Leonardo DiCaprio, the United Nations’ newly appointed Messenger of Peace, addressed world leaders at Tuesday morning’s UN Climate Summit, urging them to fully address climate change, “the greatest challenge of our existence on the planet.”

DiCaprio touched on his history as an actor to differentiate from fiction and fact.

“I play fictitious characters often solving fictitious problems. I believe that mankind has looked at climate change in that same way,” he said. “As if pretending that climate change wasn’t real would somehow make it go away, but I think we all know better than that now . . . None of this is rhetoric, and none of it is hysteria. It is fact.”

DiCaprio also joined some 400,000 people in New York City in a Sunday march meant to raise climate change awareness.